172 Geophysics in Geothermal Exploration 5.3.4 Current transmissions The transmissions have been designed according to two approaches, based on the dual need to reach deep layers and, at the same time, to have a fair near surface detail. A first series of 32 transmissions was carried out with the aim of achieving the maximum sensitivity of the measurements at great depths for the different receivers. For these currents’ injections, a fixed pole “A” in TX1 and a second mobile transmitter “B” with a “forward” acquisition scheme was used: TX1-2, TX1-3, TX1-4, ..., TX1-23 (Figure 5.7). The injection dipole TX1-23 has the maximum aperture of about 5.5 km. Figure 5.7 Location of the transmission’s electrodes on main central axe. Pole A was in TX1 and pole B was moving on all TX from TX2 to TX23. A backward acquisition was done on the main axe, with pole A fixed at TX23 and pole B moving backward on TX21, TX19, TX17, …, TX1 (one out of two). This acquisition sequence was followed by a backward energization scheme with fixed pole “A” in TX23: TX 23-21, TX23-19, TX23-17, ..., TX23-1 (Figure 5.7). The combination of these transmission dipoles schemes allowed to acquire (i) dipole-dipole protocol arrays, when the transmitting dipole is very far from the receiving dipole, (ii) pole-dipole, when the mobile transmitter “B” is close to the receiving dipole, and, (iii) gradient dipole, when both transmitters “A” and “B” are “external” and far from the receiving dipole. The second group of transmissions (72 dipole combinations) involved electrodes with numbering from TX24 to TX50, each of which has been combined in sequence with three different “remote” transmission poles, fixed respectively at the TX9, TX13 and TX15 electrodes (Figure 5.8). This approach enabled us to achieve high-sensitivity measurements across the area of receivers, effectively covering both shallow and medium investigation depths.
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